ttl output to cmos input

[killabyte]

i need to input signal to a cmos device, but it is shielded and fed from 12vdc and have an internal regulator to 2.9vdc.

i have a 12vdc supply and a 5v regulator.

how to convert the 5vdc signal from mcu to input the device.

its input is very sensitive to overcharge so 3.3vdc would destroy it.

any tip any sug?

best

killa

 

[buzzp]

Maybe I am missing something but can't you turn on a transistor with the mcu signal which would allow your 2.9V to pass through the transistor to the CMOS device? You could protect the input from higher voltages with a zener.

 

[killabyte]

yap but ill need a 2.9 source, altough it could be a zener made one, ive seen another method before, but i cant find my paper.

some day i read about problems involved with inappropiated level shifting.

thanks

killa

 

[nbucska]

What are the TTL and CMOS devices ( maker & part#)
and the highest data rate? I have to look up their data sheets.


<nbucska@pc33peripherals.com> omit 33 Use subj: ENG-TIPS
Plesae read FAQ240-1032

 

[felixc]

Your information is quite confused. If the CMOS input is sensitive, and that the frequency of the signal low enough, a series resistor will prevent destroying the device.

 

[elfgriper]

killabyte,
You might be able to use a single transistor to convert the logic 5v by driving the base with the logic (5v) then using the open collector with a resistor tied to the 12 or 5volt ps with a zener to set the 2.9volt level to the cmos.
-elf

 

[elfgriper]

killabyte,
by the way this would be an inverted signal so if that;s a problem you might have to use a second transistor to get back to the original non inverted form.
-elf

 

[MrBananas]

Why not put a 2.9V signal zener across the CMOS input and just feed it with the TTL output through a series resistor to limit the current through the zener?

Another possibility might be to use a voltage divider network with maybe a 3.1V zener for protection only. The input impedance on CMOS inputs is very high so I would think that might work.

 

[killabyte]

thanks guys, i yesterday i found the paper ive lost.

usually converting from 5v ttl to 3 v cmos logic requires only a 10K series resistor if the cmos gate is diode protected (diode from gnd to input and diode prom input to 3v supply)

to achieve bidirectional comm, is common the use of logic level translators as found in maxim.com

thanks all for your valuable help

killa

 

[buzzp]

Just dont depend on the diode in the device, generally. These are rather weak and depending on the current levels could blow this internal diode. Externals are suggested.